wav: init st to NULL to avoid a false-positive warning.

If st is NULL, it means no 'fmt ' tag is found, but 'data' tag (which
needs a previous 'fmt ' tag to be parsed correctly and st initialized)
check will make sure st is never dereferenced in that case.

Fixes warning:
    libavformat/wav.c: In function ‘wav_read_header’:
    libavformat/wav.c:499:44: warning: ‘st’ may be used uninitialized in this function [-Wmaybe-uninitialized]
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavformat/wav.c

@@ -387,7 +387,7 @@ static int wav_read_header(AVFormatContext *s)
     int rf64;
     uint32_t tag, list_type;
     AVIOContext *pb = s->pb;
-    AVStream *st;
+    AVStream *st = NULL;
     WAVContext *wav = s->priv_data;
     int ret, got_fmt = 0;
     int64_t next_tag_ofs, data_ofs = -1;